IMPACT OF ANTIRETROVIRAL THERAPY ON LEVELS OF LYMPHOCYTE APOPTOSIS

The benefits of combination antiretroviral therapy were demonstrated by improved survival and fewer opportunistic infections.¹⁰¹ These outcomes were associated with improvement in immuno­logic dysfunction, such as reduced markers of activation and improved CD4 T lymphocyte response to recall antigens.¹⁰² Despite this, several markers of defective cellular immune responses associated with HIV infection persist, despite the use of highly active antiretroviral therapy (HAART).¹⁰³ A large number of studies have suggested that levels of apoptosis are decreased in association with HAART, and these are summarized in Table 21.3.

TABLE 21.3

Relationship of Lymphocyte Apoptosis Levels to Antiretroviral Therapy

T cell but not CD8 T cell or

PBMC apoptosis

TABLE 21.3 (Continued)

Relationship of Lymphocyte Apoptosis Levels to Antiretroviral Therapy

(continued)

TABLE 21.3 (Continued)

Relationship of Lymphocyte Apoptosis Levels to Antiretroviral Therapy

CD4 and CD8 T cells

TABLE 21.3 (Continued)

Relationship of Lymphocyte Apoptosis Levels to Antiretroviral Therapy

An important source of information included analysis of apoptosis in vivo.

The majority of studies have, however, examined apoptosis ex vivo in PBMC or in lymphocyte subsets. The parameter measured was most often spontaneous apoptosis, but Fas-mediated apoptosis or activation-induced apoptosis was also measured. In elegant studies using deuterated glucose to label DNA in proliferating cells and ex vivo analysis of PBMC, followed by mathematical modeling, Mohri and colleagues demonstrated that antiretroviral therapy reduced both proliferation and apoptosis in CD4 T lymphocytes in association with increases in CD4 T lymphocyte numbers.¹⁰⁹ This finding is in agreement with the majority of the 40 studies outlined in Table 21.2. Most of these studies show decreased levels of apoptosis either in PBMCs or, more specifically, CD4 and CD8 T lymphocytes. In most cases, this has occurred in response to protease inhibitor (PI)-based therapy.

Not all studies confirm a beneficial effect of HAART on apoptosis. A number show no reduction in the level of apoptosis by HAART or persistent elevation of apoptosis for long periods after HAART initiation.^{115,121,125,126} Elevated sFas levels may not be corrected by up to a year of HAART.¹²⁷ The reason for these differences is not immediately obvious, as similar parameters of apoptosis were measured in many studies. One source of variation is that intercurrent illnesses may cause transient elevations in apoptosis, just as they may cause viral blips.¹⁰⁴ The effect of these may be quite marked in series containing small numbers of individuals. Another potentially confounding variable is the effect of antiretroviral therapy, as discussed elsewhere in this book. AZT or 3TC may enhance apoptosis,^121,128 although a further study has suggested that AZT and 3TC may be associated with a more marked decrease in apoptosis than AZT and ddI.¹²⁹ The nonnucleoside reverse transcriptase inhibitor (NNRTI) efavirenz may enhance caspase activation and mitochondrial membrane perme- abilization in vitro.¹³⁰ Conversely, the protease inhibitors ritonavir, saquinavir, and nelfinavir were shown to decrease apoptosis in lymphocytes and bone marrow progenitors in vitro.^131,132 PIs may prevent activation-associated mitochondrial hyperpolarization.^132,133 Differences may exist between PIs, as evidenced by reports that Indinavir does not inhibit apoptosis.^134,135 In one of these studies, low doses of Indinavir helped promote lymphoproliferation in vitro,¹³⁵ whereas in a second study, using higher doses, lymphoproliferation was blocked.¹³⁴ Although these findings could be interpreted as being due to differences between PIs of different structures, one of these studies also tested Saquinavir and the other Nelfinavir with similar results.^134,135 One of these studies used short-term culture, and this might translate into a later inhibition of apoptosis.¹³⁴ The other study involved up to 14 days of in vitro culture with PIs before apoptosis assessment and only detected reduced apoptosis with the highest dose of each PI, administered for 14 days to PBMCs from HIV-seropositive but not from HIV-seronegative individuals.¹³⁵ The exact reasons for the different findings are unclear.¹³⁶ However, a review of individuals receiving postexposure prophylaxis was unable to detect any alteration in apoptosis with AZT; AZT plus 3TC; or AZT, 3TC, plus Indinavir,¹³⁷ whereas a different study of postexposure prophylaxis showed reduced susceptibility to apoptosis after Nelfinavir, AZT, and 3TC.¹³⁸ This study does not exclude the possibility of an indirect effect on lymphocyte apoptosis in an activation-specific context, such as by HIV infection in vivo or by ex vivo culture of lymphocytes from HIV-seronegative individuals.

However, some of these studies do not involve PI-based therapy,¹¹⁰ and a study that compared PI- and NNRTI-based regimens found no differences in rates of CD4 T lymphocyte recovery or apoptosis.¹¹⁴ One interpretation for these apparently disparate findings is that direct induction of apoptosis, which kills viral-infected cells or depletes activated cells capable of becoming infected with HIV, might aid the virological response.¹⁴² Hence, this helps reduce apoptosis in bystander cells, whereas direct inhibition of bystander apoptosis, particularly in the context of suboptimal virological response by PI, might help boost CD4 T lymphocyte counts.